4, 5-imidazoledicarbonitrile and method of preparation



Patented Dec. 19, 1950 UNITED STATES PATENT OFFICE 4,5-IMIDAZOLEDIUARBONITRILE SAND METHOD OF PREPARATION David W. Woodward, Wihnington,'Del.,.assignor to E. I. du Pont de Nemours and Company, WilmingtongDeL, a corporation ofDelaware No'Drawing. Application August 30, 1949, Serial No. 113,247

ical reactions, to be either (I) 1,2-dicyano-L'2- diaminoethylene (i. *e., diaminomaleonitrile) or (II) l,2-dicyan 1:amino-Z-iminoethane (i. e., aminoiminosuccinonitrile) or an equilibrium mixture of the two forms:

hydrogen cyanide rtoitstetramer in essentially 'quantitative'yields, has been described in my co- "pending application'serial No..10;567, filed February 24, 1948, now Patent No. 2,499,411. This method comprises continuously passing liquid .r-hydrogen cyanide at-aitemperature of 0100 C.

into contact with a water-insoluble, -basicr-react- 3 -ing, .solidcatalyst, e. g. alumina, and rapidly removing'the tetramer, as it is formed, from the catalyst.

It is an object of this invention to produce new nitrogen-containing \heterocyclic compounds. rI-tds'anothereobjeot of this inventionto provide new and improved processes .for the production of nitrogen-containing 'heterocyolic compounds.

vvIti'is ratmore specific "object of this invention to produce :4,- imidazoledicarbonitrile and the 2- ;"hydrocarbonsubstituted derivatives thereof, and

: -toprovi'de new ianduseful processeslfor the production thereof.

'Otherobjects of the invention will appear herei 7 inafter.

The objects of this invention may the accomplished, in generaLebyheating to a temperature of -180 C. hydrogen cyanide tetrame'r together aith'an ortho ester of a monocarboxylic acid.

2 :This 1. reaction maybe illustrated by the following equation. ('A) N-C-CN /3 4. C4H4N4'+"R-CE(OR'); R-og\ ll sa on 'N- CN flydrogen Orthmester ilmidazole derivative 1 cyanide tetram'er ,(III) 'R. in the above equation is hydrogen-"or a monovalent'hydrocarbon radical,prferablyrdlkfl radicals,- particularly of from one to 'elevenicar- 'bon atoms, and R' is alower'alkyl radical, zpreferably" of 'less than three carbon atoms.

"The process or this invention, therefore, produces imidazoles having 'nitrile groups attached to each ofthe nuclear carbon atoms at the'designated4- and 5-positions of the imidazolenucleus, 'any 'other substituent on said nucleus being a monovalerit hydrocarbon radical "bonded on 'ithe remainingnuclear carbon-atom atthe 2-position.

'-In one method 'of'practicing the inventiomrthe ortho ester, for example, ethyl orthoformate'rand hydrogen cyanide tetramer, preferably :ine uimolar amounts'arfd in the presence of aninert *liquid-solvent mediumisuch as, for exampleisubstantially water-free anisole, are heated, tsuitably with stirring, at atemperature of'about 70- '100 "C. until at "least 160%, and preferably 60- r of the alcohol derived :from "the condensa- 'tion is removed by distillation, and then increasin the temperature 5130 -.180" (3., preferably -160" C., where it is maintained for "about 10-30 minutes to complete'the condensation. Although not essentiaLit is preferable to complete the -condensation iat"--the"higher temperature in the pr-esence of -a small'ainount, i.'e., 0Z1'% to*110'% of 'the weight o'f the hydrogen cyanide tetramer employed, of an alkaline agent. Suitable-agents for this use arealkalimetal alcoholates, 'for example, sodium'methoxide. The presenceofltliese alkaline "agents' at the start of the reaotioniis,

however, undesirable because they promote ide- "composi-tionof "the hydrogen cyanide tetramer.

The method of isolation will varyydepending "on the inertliqui'd medium employed :and the "particular im'idazole pro'duced. Aside from ithe removal 'of the liquid reaction medium, v the 2' con- "densa'tion prm'iuct can, :in many instances, be

isolated by cooling to cause crystallization eoriiby precipitation with a solvent such as hexane or any common organic solvent in which the product is insoluble. The solid imidazole can, in general, be purified by recrystallization from water, diethyl ether, benzene, etc.

In the preparation of the e,5-imidazoledicarbonitrile and its Z-hydrocarbon substituted derivatives, it is preferable to employ equimolar amounts of hydrogen cyanide tetramer and ortho ester, since excesses over these amounts, for example over molar excess, of the ortho esters can react further with the hydrogen on the nuclear nitrogen atom in the 1-position of the above-named imidazole compounds to split out alcohol as illustrated by the following equation:

RC=(OR')2 wherein R. and R have the same meaning as previously designated in Equation A. The compounds illustrated by the Formula IV above also form a part of this invention. They can be readily prepared in good yields by reacting the hydrogen cyanide tetramer with at least double the equimolar amount of the ortho esters.

Although it is not absolutely essential, it is preferable to conduct the condensation in the presence of an inert medium capable of dissolving the reactants because it results, in general, in better reaction by promoting initimate contact of the reactants, and also serves as a medium for distillation of the alcohol of condensation. This liquid medium should be inert toward the reactants employed in the condensation and boil at least 20-30" C. above the alcohol of condensation. Preferably, it should be substantially free, i. e., less than about 0.1%, of water and act as a solvent for the reactants. Examples of suitable inert solvent liquids, include anisole,

phenetole, dibutyl ether, dioxane, toluene, and xylene. Of these, the ethers are preferred. Although the amount employed can vary widely,

' depending on the particular solvent used and the compounds processed, excellent results are obtained with from five to twenty-five parts of inert liquid solvent medium to each part of tetra mer.

The following examples illustrate, in detail,

certain preferred methods for the production of the above-mentioned new compounds. at is to be understood that the details of the examples are illustrative and are not to be taken as limitative of the invention.

Eacample I To a closed reactor equipped with an agitator and a distillation column is charged 200 parts of substantially water-free anisole, 20 parts of hydrogen cyanide tetramer, and 29 parts of ethyl orthoformate. The mixture is then stirred and heated to distill oiT the ethanol (still-head temperature, 80 C.). After 18 parts of ethanol has distilled, 0.4 part of sodium methoxide is introduced into the reaction mixture and distillation is continued until the distilling temperature has been maintained at about 150 C. for -20 minutes. The hot solution is filtered and then cooled, whereupon a white solid crystallizes. A solution of this solid in 50 parts of boiling water is clarified with activated charcoal, filtered and cooled to yield 17.5 parts of white crystalline 4,5-

Carbon Hydrogen Nitrogen Per cent Per amt Per cent Calculated for C5HaN4. 50. l. 70 47. 4? Found by analysis 51. 42 2. 05 47. 14 Found by analysis 51. 48 2. 27 1. ll.

Example II A stirred mixture of parts of dioxane, 5 parts of hydrogen cyanide tetramer, and '7 parts of ethyl ortholormate is heated in a closed reaction vessel equipped with a distillation column until the distillation temperature remains constant for about 10 minutes, and about 50 parts of distillate is collected. The reaction mixture is then diluted with 35 parts of hexane and cooled to crystallize. The precipitated solid is filtered and recrystallized from diethyl ether to give 5 parts of white crystals, M.P. -137 C. This compound results from the elimination of two chemical equivalents of ethanol between one of hydrogen cyanide tetramer and one of ethyl orthoformate. The structure is believed to be either Carbon Hydrogen Nitrogen Percent Perce n! Peru 1 Calculated for C7H8N4O 51. 22 4. 87 34. 00 Found by analysis 51. 38 4. 93 33. 78 Found by analysis 51. 40 4. 80 33. 61

By dissolving the above compound in boiling water and then cooling, hydrogen cyanide tetramer is recovered.

Example III A mixture of one part of the compound of Example II, Cvl-I8N4O, and 45 parts of xylene is boiled 15 minutes, cooled to bring about crystallization, filtered, and dried. The resulting white solid is recrystallized from diethyl ether to give 0.6 part of 4,5-imidazoledicarbonitrile, lVLP. fi l- (3., identical with the product of Example I.

Ewam'ple IV A mixture of 2 parts of the compound of Example II, C'THBNO, is boiled with 25 parts of amaesaasr sole and 0.05part ofsodiummethoxide. The reaction is cooled and the white crystalline solid is filtered and recrystallized from diethyl ether to yield 1.8 parts of 4,5-imidazole-dicarbonitrile identical with that ofExample I.

Example V Into a; closed reactor equipped with distillation column and an agitator is placed parts of ethyl orthoiormate, Z'parts of hydrogen cyanide tetramerand 0.1 part of potassium carbonate. The mixture is then stirred andheatedat 80 C. to distill ofi'the" alcohol after which heating is continued until the distillation temperature is increased to 140 C. OILdfllltlOn of the cooled reaction mixture with 100 parts of hexane, the white crystalline. product which precipitates is filtered and recrystallized from hexane to give two parts of whitafiuffy needles, MP; 55-60" C. This compound, the result or reaction between two'mol'es' of ethyl orthoformate andone mole of hydrogen cyanide tetramer, is believed to be 1- (diethoxymethyl) 4,5-imidazoledicarbonitrile, g on zmoz. (R, is hydrogen and R is ethyl in previously mentioned Formula IV.)

Chemical analysis of the whiteproduct for car bon, hydrogen andv nitrogen, and comparison with the calculated percentages of carbon, hydrogenand' nitrogen inCmHrzNeOz gave the fol lowing results:

Carbon vHydrogen Nitrogen Percent Percent Percent Calculated for CiofluNlOr; 45

.Eound by analysis 53. 22 5. 25 1 27. 60

Found by analysis 53. 50 5. 14 27; 23

Example VI Lcoal, filtered, cooled, and then treated with 6.5

parts of concentrated hydrochloric acid to precipitate pure-White crystalline 4,5-imidazol'edicarbonamide, C5H'sN4O2, which sublimes without 'rnelting at. above 300 C., but melts in a sealed tube at 370" C.

Chemical analysis of the white crystalline product for carbon, hydrogen and nitrogen and comparison with the calculated percentages of carbon, hydrogen and nitrogen in CsHsNiOz gave the following. results:

Carbon Hydrogen Nitrogen Percent Percent Percent Calculatedfor CkHfiNOL V" 38. 96 3. 90. 36. 36 Found byanalysisn 38. 74 3. 85 36. 50 :Eonndby analysis 39. 06 3.90 36. 66

Example VII The diamide described in Example VI can be converted to, xanthine by the l-lofmann hypohalite rearrangement. This conversion has been reportedbyg Baxter and Spring (J; Chem. Soc, 1945,

5 232) in yields of ten per cent. It has now been found that by use of the following carefully selected conditions yields-of xanthine in excess or 60 per cent can becbtained.

One (1) molar equivalent of 4,5-imidazoledi' carbonamide and 3 to 5 molar equivalents of sodium hydroxide are-dissolved in 5 to 10 times their combined weight of warm (50 C.) water. To the resulting cooled (-3 C.) solution there isadded at 3-" C. to 0 C. with rapid agitation- 1.251 to 1.75 molar equivalents of sodium hypochlorite: as a 0.2 N to 1 N solution during 0.1 to 1.0 hour; After addition is complete, the solution, which is now a yellow color, is warmed to 60 C., neutralized with acetic acid, cooled and filtered. The resulting crude. xanthine together with: twice its weight of sodium bicarbonate is dissolved in 25 to 50: times its weight of water; The resulting solution is. then clarified with antivated charcoal and filtered. Neutralization of the cooled filtrate withdilute (1:10) hydrochloric acid precipitates. pure xanthine which, afterfiltering and drying, is obtained as a white powder in a yield of 60 to'70 per cent.

Example VIII Methylation of the xanthine of Example VII with dimethyl sulfate, as for example,nb'y the method of M. D. Rubtsov EC. A..39,.697 (1945) J. Gen. Chem..U. S. S. R. 1'3, 710 (1943)], yields caffeine as pure-white crystals. melting at 233'- 286 C. in a sealedtube heated at the rate of 2 C. per minute. By comparison, U. S. P. flake cafleine melts at 232235 C. under the same conditions, and a mixture of the. two melts at 233-236 C. Chemicalanalysis of the compound for carbon, hydrogen and nitrogen, and comparison with the calculated percentages of these elements in caffeine: gives the following results:

Garb on I Hydrogen Nitrogen Per cent Per cent Per cent Calculated for CaH'mNQOz HU 49.50 5. l6 28. Found by analysis 49. 74 5. 19 28. 63 Found hy'analysis 49. 55 5. 06 28.73

Example. IX

Employingtheimethod described in ExampleI and replacing the ethyl orthoformat'e used therein with 31 parts of ethyl orthoacetate, the product obtained is 2-methyl-4.5 imidazoledicarbonitrile, CsH4N4, M. P. 230-231'C.

Employing the method described in Example I and replacing the ethyl orthoformate employed therein with 32 parts of ethyl orthopropionate, the product isolated is 2-ethyl-4.5imidazoledi carbonitrile',C'1HeN4, M. P. 184-187 C;

Carbon Hydrogen Nitrogen Per cent I Per cent Per cent Calculated for C7HfiN-l 57. 54 4. 10 38. 35 Found by analysis... 57. 92- 4. 28 37. 94 Found analysis 5 7. 33 4.12 37. 73

The process of this invention is not limited tothe specific: QllihO esters disclosed in theforegoing examples, but embraces more: broadly the ortho esters of 'monocarboxylic acids, 1. e.; tri-.:

esters of ortho monocarboxylic acids (see Introduction to the 1945 Chemical Abstracts Index, paragraph 285), particularly those wherein the ester function, e. g., the trialkoxymethyl group, is attached to a monovalent hydrocarbon radical. Other suitable ortho esters are methyl orthoformate, methyl and ethyl orthoacetates, methyl and ethyl orthopropionates, methyl orthobutyrate, methyl orthovalerate, methyl orthocaproate, methyl orthoisocaproate, methyl orthododecanoate, ethyl orthophenoxyacetate, ethyl orthobenzoate, and ethyl orthophenylacetate. Many of these compounds and the method for their preparation are described by Brooker and White, J. Amer. Chem. Soc. '7, 2485 (1935). Of these, it is preferred to employ ortho esters of alkanoic acids of one to twelve carbon atoms, particularly lower tria lzyl esters in which the alkyl radical is of not more than two carbons because, by use of these, the alcohol of condensation is readily removed. The foregoing ortho esters are employed in accordance with procedures described herein. For example, by replacing the ethyl orthopropionate in Example VII with methyl orthocaproate, 2-pentyl-4,S-imidazoledicarbonitrile may be produced.

Other alkaline agents which can be satisfactorily employed to facilitate completion of the condensation reaction after removal of most of the alcohol of condensation are sodium and potassium ethoxide, sodium tertiary-butoxide, sodamide, sodium, potassium, sodium and potassium hydroxides, sodium carbonate, and the like.

The compounds of this invention are important intermediates in the synthesis of purines, especially xanthine and its methylation products such as cafieine, theobromine, theophylline, etc. For instance, the 4,5-imidazoledicarbonitrile described herein is an important new raw material for the preparation of the corresponding dicarbonamide which, as has been shown in Examples VI, VII and VIII, is of particular interest for the synthesis of caifeine of high purity. This dicarbonamide is also a useful resin intermediate that reacts with formaldehyde to give a clear, colorless, insoluble resin. The 4,5-imidazoledicarbonitrile products of this invention are also useful as intermediates for the preparation of dyes, pigments, and, surface-active agents.

Reference in the specification and claims to parts, proportions and percentages, unless otherwise specified, refers to parts, proportions and percentages by weight.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I claim:

1. A process for the production of 4,5-dicyanoimidazoles which comprises heating to a temperature of 70 C. to 180 C. hydrogen cyanide tetramer together with an ortho ester of a monocarboxylic acid.

2. A process for the production of 4,5-dicyanoimidazoles which comprises heating to a temperature of 70 C. to 180 C. a solution of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid in an inert liquid.

tetramer together with an ortho ester of a monocarboxylic acid having the formula wherein R is a substituent of the group consisting of hydrogen and monovalent hydrocarbon radicals of from 1 to 11 carbon atoms and R is an alkyl radical of less than 3 carbon atoms.

5. A process for the production of 4,5-di-cyanoimidazoles which comprises heating to a temperature of C. to 180 C. a solution, in an inert liquid medium, of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid having the formula wherein R. is a substituent of the group consisting of hydrogen and alkyl radicals of from 1 to 11 carbon atoms and R is an alkyl radical of less than 3 carbon atoms.

6. A process for the production of 4,5-dicyanoimidazoles which comprises dissolving in an inert liquid medium hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid having the formula R--CE(OR)3 wherein R is a substituent of the group consisting of hydrogen and alkyl radicals of from 1 to 11 carbon atoms, and R. is an alkyl radical of less than 3 carbon atoms, heating said solution to a temperature of 70 C. to C. until at least 60% of. thealcohol formed by condensation is re moved from the solution, then heating said solution to a temperature of C. to C. for a period of about 10 to 30 minutes.

7. A process for the production of 4,5-dicyanoimidazoles which comprises dissolving in an inert liquid medium hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid having the formula wherein R is a substituent of the group consisting of hydrogen and alkyl radicals of from 1 to 11 carbon atoms, and R is an alkyl radical of less than 3 carbon atoms, heating said solution to a temperature of 70 C. to 100 C. until at least 60% of the alcohol formed by condensation is removed from the solution, adding an alkaline agent, then heating said solution to a temperature of 120 C. to 180 C. for a period of about 10 to 30 minutes.

8. A process for the production of 4,5-dicyanoimid-azoles which comprises dissolving in an inert liquid medium hydrogen cyanide tetramer and an ortho ester Of a monocarboxylic acid having the formula RCE (OR') 3 wherein R is a substituent of the group consisting of hydrogen and alkyl radicals of from 1 to 11 carbon atoms, and R is an alkyl radical of less than 3 carbon atoms, heating said solution to a.

temperature of 70 C. to 100 C. until at least 60% of the alcohol formed by condensation is removed from the solution, adding 0.1% to 10%, based on weight of hydrogen cyanide tetramer dissolved, of an alkaline agent, then heating said solution toa temperature of 120 C. to 180 C. for a period of about 10 to 30 minutes.

9. A process for the production of compounds taken from the group consisting of 4,5-imidazoledicarbonitrile and Z-hydrocarbon substituted 4,5- imidazoledicarbonitriles in which the hydrocarbon substituent contains 1 to 11 carbon atoms which comprises heating to a temperature of 70 C. to 180 C. about equi-molecular amounts of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid. 10. A process for the production of compounds taken from the group consisting of 4,5-imidazoledicarbonitrile and 2-hydrocarbon substituted 4,5- imidazoledic'arbonitriles in which the hydrocarbon substituent contains 1 to 11 carbon atoms which comprises heating to a temperature of 70 C. to 180 C. about equimolecular amounts of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid of not to exceed 12 carbon atoms.

11. A process for the production of compounds taken from the group consisting of 4,5-imidazoledicarbonitrile and Z-hydrocarbon substituted 4,5- imidazoledicarbonitriles in which the hydrocarbon substituent contains 1 to 11 carbon atoms which comprises heating to a temperature of 70 C. to 180 C. about equimolecular amounts of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid having the formula wherein R is a substituent of the group consisting of hydrogen and monovalent hydrocarbon radicals of from 1 to 11 carbon atoms and R is an alkyl radical of less than 3 carbon atoms.

12. A process for the production of compounds taken from the group consisting of 4.5-imidazoledicarbonitrile and Z-hydrocarbon substituted 4,5- imidazoledicarbonitriles in which the hydrocarbon substituent contains 1 to 11 carbon atoms which comprises heating to a temperature of 70 C. to 180 C. about equimolecular amounts of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid having the formula R-CE OR) 3 wherein R is a substituent taken from the group consisting of hydrogen and alkyl radicals of from 1 to 11 carbon atoms, and R is an alkyl radical of less than 3 carbon atoms.

13. A process for the production of compounds taken from the group consisting of 4,5-imidazoledicarbonitrile and 2-hydrocarbon substituted 4,5- imidazoledicar-bonitriles in which the hydrocarbon substituent contains 1 to 11 carbon atoms which comprises dissolving in an inert liquid medium about equimolecular amounts of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid having the formula wherein R is a substituent of the group consisting of hydrogen and alkyl radicals of from 1 to 11 carbon atoms, and R is an alkyl radical of less than 3 carbon atoms, heating said solution to a temperature of 70 C. to 100 C. until at least 60% of the alcohol formed by condensation is removed from the solution, then heat ng sai 1 solution to a temperature of 120 C. to 180 C. for a period of about to minutes.

14. A process for the production of compounds taken from the group consisting of 4,5-imidazoledicarbonitrile and Z-hydrocarbon substituted 4,5- imidazoledicarbonitriles in which the hydrocarbon substituent contains 1 to 11 carbon atoms which comprises dissolving in an inert liquid medium about equimolecular amounts of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid having the formula wherein R is a substituent of the group consisting of hydrogen and alkyl radicals of from 1 to 11 carbon atoms, and R is an alkyl radical of less than 3 carbon atoms, heating said solution to a temperature of 70 C. to C. until at least 60% of the alcohol formed by condensation is removed from the solution, adding an alkaline agent, then heating said solution to a temperature of C. to C. for a period of about 10 to 30 minutes.

15. A process for the production of compounds taken from the group consisting of 4,5-imidazoledicarbonitrile and 2-hydrocarbon substituted 4,5- imidazoledicarbonitriles in which the hydrocarbon substituent contains 1 to 11 carbon atoms which comprises dissolving in an inert liquid medium about equimolecular amounts of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid having the formula wherein R. is a substituent of the group consisting of hydrogen and alkyl radicals of from 1 to 11 carbon atoms, and R is an alkyl radical of less than 3 carbon atoms, heating said solution to a temperature of 70 C. to 100 C. until at least 60% of the alcohol formed by condensation is removed from the solution, adding 0.1% to 10%, based on the weight of hydrogen cyanide tetramer dissolved, of, an alkaline agent, then heating said solution to a temperature of 120 C. to 180 C. for a period of about 10 to 30 minutes.

16. A process for the production of compounds taken from the group consisting of 4,5-imidazoledicarbonitrile and Z-hydrocarbon substituted 4,5- imidazoledicarbonitriles in which the hydrocarbon substituent contains 1 to 11 carbon atoms which comprises dissolving in an inert liquid medium about equimolecular amounts of hydrogen cyanide tetramer and an ortho ester of a monocarboxylic acid having the formula wherein R is a substituent of the group consisting of hydrogen and alkyl radicals of from 1 to 11 carbon atoms, and R is an alkyl radical of less than 3 carbon atoms, heating said solution to a temperature of 70 C. to 100 C. until at least 60% of the alcohol formed by condensation is removed from the solution, adding 0.1% to 10%, based on the weight of hydrogen cyanide tetramer dissolved, Of sodium methylate, then heating said solution to a temperature of 120 C. to 180 C. for a period of about 10 to 30 minutes.

17. 4,5-imidazoledicarbonitrile.

DAVID W. WOODWARD.

REFERENCES CITED The following references are of record in the file of this patent:

Chem. Abst., vol 31, page 6654 (1937).

Chem. Abst., vol. 32, pp. 3394-3395 (1938) 

3. A PROCESS FOR THE PRODUCTION OF 4.5-DICYANOIMIDAZOLES WHICH COMPRISES HEATING TO A TEMPERATURE OF 70*C. TO 180*C. HYDROGEN CYANIDE TETRAMER TOGETHER WITH AN ORTHO ESTER OF A MONOCARBOXYLIC ACID HAVING THE FORMULA R-C$(OR'')3 WHEREIN R IS A SUBSTITUENT OF THE GROUP CONSISTING OF HYDROGEN AND MONOVALENT HYDROCARBON RADICALS OF FROM 1 TO 11 CARBON ATOMS AND R'' IS AN ALKYL RADICAL OF LESS THAN 3 CARBON ATOMS. 